We propose to develop an in-vitro x-ray nano-beam irradiation system with 30 nm beam size and an integrated multi-mode visible-light microscope to precisely locate and irradiate sub-cellular organelles. This instrument is a valuable analytical tool for the Radiation Research Program at the National Cancer Institute. We expect it will be used for cellular-level and tissue-level studies in a number of areas including: understanding the pathways of carcinogenesis from radiation exposure and self repair mechanisms; developing chemo-prevention drugs to prevent or delay adverse effects from radiation exposure; improving radiation treatments that can potentially combine radiotherapy, chemotherapy, and immunotherapy; and establishing target based strategies to enhance tumor sensitivity and protect normal tissue during radiotherapy. The proposed system improves upon existing micro-beam irradiation systems by offering about one order of magnitude finer irradiation resolution and accuracy. Since the x-ray beam is concentrated into an extremely fine focal point, a rad-level dosage can be delivered with microsecond exposures. This allows a very high throughput. Finally, by using an economical micro-focused x-ray tube source, the proposed system can be made available to a wide range of research and industrial institutions at a modest cost.